The present invention relates to twin sheet forming and more particularly, to a system and method for twin sheet forming using an in-line machine and plug assist.
Many articles that have traditionally been made of metal or wood are now produced from thermoplastic materials. Forming automotive fuel tanks, for example, from thermoplastic materials provides a number of advantages. The fuel tank can be made in virtually any shape to fit in any available space within the automobile, thereby maximizing the use of space. Using the thermoplastic material, the automotive fuel tank can be made leak proof and more flexible, preventing the fuel tank from rupturing. Using thermoplastics and thermoforming techniques is also advantageous for other hollow or partially hollow articles.
One method for making hollow thermoplastic articles from thermoplastic material is a conventional blow molding technique. Some types of hollow articles, however, require an insert to be installed within the hollow article, and blow molding cannot be used to do this. Automotive fuel tanks, for example, often require baffles to be formed within the tank.
Twin sheet forming is another method of forming hollow articles from thermoplastic material. According to conventional twin sheet forming methods, first and second sheets of thermoplastic material are heated and thermoformed using first and second thermoforming tools, such as vacuum molds, positioned opposite one another. Before the sheet surfaces facing away from the tools have cooled, the tools are moved together or closed so that the hot surfaces of the sheets fuse together proximate the edges or in any other area that protrudes and contacts the opposing sheet. The residual heat and soft surface of the thermoplastic materials allows the sheets to fuse together.
One type of conventional twin sheet thermoforming system 10, FIG. 1, uses a rotary machine to form first and second sheets 12, 14 into a hollow article 16. The rotary machine has four stations 20, 22, 24, 26 and uses four clamp frames to rotate and index the sheets 90xc2x0 to each of the stations. The first and second sheets 12, 14 are loaded at an load/unload station 20, for example, by clamping the sheets 12, 14 with clamp frames. The clamp frames are then indexed such that the first and second sheets 12, 14 are transferred respectively to first and second heating stations 22, 24. Once the sheets 12, 14 have been heated, both of the sheets 12, 14 are indexed into the forming station 26 in a rapid sequence.
The forming station 26, FIGS. 2-4, includes upper and lower platens 28, 30 that support first and second thermoforming tools 32, 34 respectively. The first sheet 12 is formed in the first thermoforming tool 32 (FIG. 2) and the second sheet 14 is formed in the second thermoforming tool 34 (FIG. 3). The upper and lower platens 28, 30, FIG. 4, are then moved together to close the thermoforming tools 32, 34 and fuse the first and second thermoformed pieces 12xe2x80x2, 14xe2x80x2 proximate the edges to form the article 16. The article 16 is then indexed to the load/unload station 20 for unloading.
One of the problems with this conventional twin sheet thermoforming system is in the timing of the rotary machine. Timing is critical in that the thermoformed parts 12xe2x80x2, 14xe2x80x2 must be brought together before the surface cools in order to properly fuse the pieces. Thinner sheets will typically cool off faster, and the slow action of the conventional rotary machine often makes the sheet cool off to such an extent that the fusing of the two sheets together is no longer possible.
Another problem that occurs in conventional twin sheet forming systems is the thinning of the sheets as they are formed into the thermoformed pieces. When forming a thermoplastic sheet into a typical female mold, the portions of the sheet that are located in the deeper cavities of the mold become substantially thinner than other areas. Forming aids, commonly referred to as plug assists, are used to distribute or prelocate the material in the deeper areas of the mold to prevent thinning. These forming aids typically include mechanical or pneumatic plug assists that move the thermoplastic sheet to the deeper areas of the mold before the vacuum pulls the thermoplastic material into those locations.
In conventional vacuum forming systems, the forming tool and the plug assist are needed at approximately the same time of the forming process. The plug assists are thus normally located on a press platen in the forming station opposite the platen supporting the forming tool. The use of plug assists in the conventional twin sheet forming system, such as the rotary system 10 described above, presents a problem in that the upper and lower platens must be used for the opposing first and second thermoforming tools and no platen is available for the plug assist tool.
Other twin sheet forming systems have attempted to use plug assists, for example, as disclosed in U.S. Pat. Nos. 3,854,860 and 5,658,523. These systems, however, have still encountered problems with timing in that the heating and forming processes cannot be performed independently for each of the sheets. The use of a single forming station with a single upper and lower platen to form both sheets can also cause problems with the different forces or loads that need to be applied during the forming process for each sheet and during the fusing process. These systems are also unable to form a hollow article having inserts formed within the hollow article, such as the baffles formed within an automotive fuel tank.
Accordingly, a need exists for a twin sheet thermoforming system and method that is capable of properly timing the transfer of the thermoplastic sheets independently from the heating station to the thermoforming station. A need also exists for a twin sheet thermoforming system and method in which a plug assist or other forming aid can be used to prevent thinning of the sheets during thermoforming without affecting the timing or the thermoforming process. A need further exists for a twin sheet forming system and method capable of forming a hollow article having one or more inserts located within the inner region of the hollow article.
The present invention features a twin sheet forming system for forming a hollow article from at least first and second sheets of thermoplastic material. The twin sheet forming system includes first and second loading stations for receiving the first and second sheets of thermoplastic material respectively and first and second heating stations for heating the first and second sheets of thermoplastic material respectively. The system also comprises first and second thermoforming stations for thermoforming the first and second sheets respectively into first and second thermoformed pieces. The second thermoforming station also presses the first and second thermoformed pieces together to fuse the first and second thermoformed pieces together. The loading stations, heating stations, and thermoforming stations are arranged in-line such that the first and second sheets of thermoplastic material are transferred independently between the stations. The system also includes a sheet transfer mechanism for transferring the first and second sheets independently from the respective loading stations to the respective heating stations to the respective thermoforming stations.
The present invention also features a thermoforming apparatus for use in a twin sheet forming system. The thermoforming apparatus comprises first and second thermoforming stations. The first thermoforming station includes a first upper platen and a first lower platen opposite the first upper platen. A first thermoforming tool is supported by the first lower platen and a first plug assist is supported by the first upper platen. A first sheet of thermoplastic material is received between the first thermoforming tool and the first plug assist. The first thermoforming station also includes a first press mechanism for pressing the first upper platen and the first lower platen toward each other such that the first thermoforming tool and the first plug assist form the first thermoplastic sheet into a first thermoformed piece in the first thermoforming tool.
The second thermoforming station includes a second upper platen and a second lower platen opposite the second upper platen. A second plug assist is supported by the second lower platen and a second thermoforming tool is supported by the second upper platen. A second sheet of thermoplastic material is received between the second thermoforming tool and the second plug assist. The second thermoforming station also includes a second press mechanism for pressing the second upper platen and the second lower platen toward each other such that the second thermoforming tool and the second plug assist form the second thermoplastic sheet into a second thermoformed piece in the second thermoforming tool.
The thermoforming apparatus further comprises a tool transfer mechanism for transferring the first thermoforming tool and the first thermoformed piece to the second thermoforming station. The second press mechanism forces the first thermoforming tool and the second thermoforming tool together such that the first and second thermoformed pieces are fused.